FIG. 1 is an illustration of a block diagram of a MOPA (Master Oscillator/Power Amplifier) laser system 10 as is known in the prior art. The MOPA laser system 10 is used, for instance, in the area of integrated circuit lithography. In one embodiment of the MOPA laser system 10, a 193 nm ultraviolet laser beam is provided at the input port of a lithography machine/scanner 2 such as stepper or scanner machines supplied by Canon or Nikon with facilities in Japan or ASML with facilities in the Netherlands. The MOPA laser system 10 includes a laser energy control system 4 for controlling both pulse energy and accumulated dose energy output of the system at pulse repetition rates, for instance, of 4,000 Hz or greater. The MOPA laser system 10 provides extremely accurate triggering of the discharges in the two laser chambers relative to each other with both feedback and feed-forward control of the pulse and dose energy.
The main components of the laser system 4 are often installed below the deck/floor 5 on which the scanner 2 is installed. However, the MOPA laser system 10 includes a beam delivery unit 6, which provides an enclosed beam path for delivering the laser beam to an input port of scanner 2. The light source includes a seed laser generator, e.g., a master oscillator 11 and an amplifier laser portion, e.g., a power amplifier 12, described in more detail below, and which may also be an oscillator, e.g., a power ring oscillator (“PRA”), also described in more detail below. For convenience sake throughout this application the seed laser may be referred to as an MO and the amplifier laser may be referred to as a power amplifier or simply a PA, with the intent to cover other forms of seed laser arrangements and amplifier laser arrangements, such as a power ring amplifier (“PRA”), which is in fact an oscillator, i.e., a power oscillator (“PO”), together forming a MOPO, and unless expressly stated otherwise these terms are meant to be so broadly defined. The light source also includes a pulse stretcher 22.
The master oscillator 11 and the power amplifier/power oscillator 12 each include a discharge chamber 11A, 12A similar to the discharge chamber of single chamber lithography laser systems. These chambers 11A, 12A contain two electrodes, a laser gas, a tangential for circulating the gas between the electrodes and water-cooled finned heat exchangers. The master oscillator 11 produces a first laser beam 14A which is amplified, in a PA configuration by two passes through the power amplifier 12, or in the case of a PO/PRA configuration, by oscillation in the PO/PRA, to produce a second laser beam 14B as shown in FIG. 1. The master oscillator 11 includes a resonant cavity formed by an output coupler 11C and a line narrowing package 11B. The gain medium for the master oscillator 11 is produced between two elongated electrodes contained within the master oscillator discharge chamber 11A. The power amplifier 12 is basically a discharge chamber 12A and in this preferred embodiment is almost exactly the same as the master oscillator discharge chamber 11A providing a gain medium between two electrodes, but the power amplifier 12 may have no resonant cavity, unlike a PO/PRA. This MOPA laser system 10 configuration permits the master oscillator 11 to be designed and operated to maximize beam quality parameters such as wavelength stability and very narrow bandwidth; whereas the power amplifier 12 is designed and operated to maximize power output. For this reason the MOPA laser system 10 represents a much higher quality and much higher power laser light source than single chamber systems.
As noted above the amplifier portion may be configured, e.g., for two beam passages through the discharge region of the amplifier discharge chamber, or for oscillation in the cavity containing the amplifier discharge chamber, as shown in FIG. 1. The beam oscillates within the cavity containing the master oscillation chamber 11A between LNP 11B and output coupler 11C (with 30 percent reflectance) of the MO 11 and is severely line narrowed on its passages through LNP 10C. A wavelength of a laser beam emitted from the output coupler 11C is measured by a line center analysis module 7. The line narrowed seed beam is reflected downward by a mirror in the MO wavelength engineering box (MO WEB) 24 and reflected horizontally at an angle slightly skewed (with respect to the electrodes orientation) through the PA wavelength engineering box (PA WEB) 26 to the amplifier chamber 12. At the back end of the amplifier, a beam reverser 28 reflects the beam back for a second pass through PA chamber 12, or for oscillation in the PO/PRA chamber, horizontally in line with the electrodes orientation. A spectral analysis module 9 measures a bandwidth of a laser emitted from the discharge chamber 12A.
The laser system output beam pulses 14B pass from the PA/PO chamber 12A to a beam splitter 16. The beam splitter 16 reflects about 60 percent of the power amplifier output beam 14B into a delay path created by four focusing mirrors 20A, 20B, 20C and 20D. The 40 percent transmitted portion of each pulse of beam 14B becomes a first hump of a corresponding stretched pulse of an output beam pulse 14C. The output beam 14C is directed by beam splitter 16 to mirror 20A which focuses the reflected portion to point 22. The beam then expands and is reflected from mirror 20B which converts the expanding beam into a parallel beam and directs it to mirror 20C which again focuses the beam again at point 22. This beam is then reflected by mirror 20D which like the 20B mirror changes the expanding beam to a light parallel beam and directs it back to beam splitter 16 where 60 percent of the first reflected light is reflected perfectly in line with the first transmitted portion of this pulse in output beam 14C to become most of a second hump in the laser system output beam pulse. The 40 percent of the reflected beam transmits beam splitter 16 and follows exactly the path of the first reflected beam producing additional smaller humps in the stretched pulse. The result is the completed output beam 14C which is stretched in pulse length from about 20 ns to about 70 ns. A beam delivery unit (BDU) delivers the output beam 14C. The BDU may include two beam-pointing mirrors 40A, 40B one or both of which may be controlled to provide tip and tilt correction for variations beam pointing.
FIG. 2 is an illustration of an energy control block diagram 50 for the MOPA/MOPO Laser System of FIG. 1, in accordance with the prior art. FIG. 2 illustrates various control elements that control a voltage supply 52 to the MOPA laser system 10. The energy control block diagram 50 includes a static control 54, which provides a basically determined voltage anticipated to achieve an energy target 56 (if there are no other influences for which account need be made). A feed forward block 58 provides a voltage adjustment based on a trigger interval 60. Trigger interval 60 is used to compute repetition rate, shot number and duty cycle, which impact the ‘voltage input—energy output’ relationship. The voltage adjustment is computed as a function of these values. An energy servo 62 adjusts the voltage input 52 based on a calculated voltage error 64 of the previous shot. A dither cancellation 66 adjusts voltage to cancel energy changes caused by a timing dither 68. Finally, an energy dither 70 provides a periodic signal added to the voltage input 52 used to estimate the effects of voltage on MO energy, output energy, and MOPA timing. These five voltage signals are added together to generate the voltage input 52. As the laser fires, the energy 72 is measured. The energy target is subtracted from the measured energy 72 to create an energy error signal 74, which is scaled by dV/dE, the laser estimate 76 of the derivative of voltage with respect to energy. The resulting voltage error 64 is used to drive adaptation algorithms 78 which adjust some of the voltage signals in a way that minimizes either energy errors, dose errors, energy sigma, or some combination thereof.
The MOPA laser system 10 shown in FIG. 1 is an improvement on the single chamber systems, providing greater beam control, beam power, and stability than the single chamber systems. However, resolving tonal disturbances and further sharpening timing and energy control of the system can significantly improve operation.